Evolution and period change in RR Lyr variables of the globular cluster M 3

TL;DR

This study models the evolution and period changes of RR Lyrae stars in globular cluster M 3, finding theoretical period change rates that align with observed data, enhancing understanding of stellar evolution in such clusters.

Contribution

The paper presents detailed hydrodynamic models of RR Lyrae stars in M 3, linking stellar evolution with pulsation period changes and providing theoretical estimates consistent with observations.

Findings

Most RR Lyr variables have period change rates between -0.02 and 0.05 day/10^6 yr.

The average period change rate is approximately 6.0×10^{-3} day/10^6 yr.

Theoretical period change rates agree well with observed values in M 3.

Abstract

The grid of evolutionary tracks of population II stars with initial masses from $0.81M_\odot$ to $0.85M_\odot$ and chemical composition of the globular cluster M 3 is computed. Selected models of horizontal branch stars were used as initial conditions for solution of the equations of radiation hydrodynamics and time--dependent convection describing radial stellar oscillations. The boundaries of the instability strip on the Herztsprung--Russel diagram were determined using nearly 100 hydrodynamic models of RR Lyr pulsating variables. For each evolutionary track crossing the instability strip the pulsation period was determined as a function of evolutionary time. The rate of period change of most variables is shown to range within $-0.02\le\dot\Pi\le 0.05~\mathrm{day}/10^6\mathrm{yr}$. Theoretical estimate of the mean period change rate obtained by the population synthesis method is $\langle\dot\Pi\rangle=6.0\times 10^{-3}~\mathrm{day}/10^6\mathrm{yr}$ and agrees well with observations of RR Lyr variables of the globular cluster M 3.